1. Field
The disclosure relates to the field of the extrusion of elastomeric compounds more particularly intended for the manufacture of tires.
2. Description of Related Art
As is known, the manufacture of the profiled elements of which tires are formed calls to a large extent upon extrusion devices which have the function of producing strips of rubber to a determined profile. These devices are formed of a barrel and of a screw mounted to rotate inside the said barrel.
The shape of the barrel and of the screw and the way in which they are arranged are contrived such that the rotating of the screw has the effect of supplying mechanical work to the compound with a view to increasing the fluidity thereof, on the one hand, and to propelling the said mixture towards an extrusion die with a view to giving the extruded product a determined profile, on the other.
The continuous strip leaving the extrusion means can be wound onto storage means with a view to later use. The invention is more particularly concerned with the extrusion devices used for feeding directly to a device downstream of the extrusion device.
One of the problems presented by the use of an extrusion device coupled more or less directly with a downstream device intended to use the continuous strip, such as a tire building means for example, is that the output of the extrusion means becomes more or less dependent upon the throughput of the device using it.
This results in significant variations in throughput which may range from complete stoppage to maximum demand on the extrusion installation at its nominal throughput. This difficulty is exacerbated by the need also to generate a product the profile of which remains constant during the phases in which consumption rate increases or decreases.
This problem is generally resolved by arranging compensating intermediate means between the outlet of the extrusion device and a downstream device for winding the extruded product. However, these compensating means are not suitable when the extruded product is to be applied directly to a downstream winding device.
Document U.S. Pat. No. 4,428,896 describes a method allowing control over the shape and dimensions of a profiled element obtained using an extruder, then transported by a conveyer to a downstream device. The control method is based on measuring the initial rotational speed of the internal screw of the extruder and the initial rotational speed of the roller, on measuring the thickness of the extruded profiled element as it leaves the extruder and as it leaves the conveyer, and on the linear weight thereof. The values of the rotational speed of the extruder screw and that of the roller are then each adjusted by their respective regulating unit on the basis of the measurements performed on the extruded profiled element, some distance away from the outlet of the extruder. Such a control method has the effect of introducing a delay into the regulation and is not suitable when the profiled element is to be applied directly to a downstream device situated at the outlet of the extruder.
One solution to this problem has been supplied by document FR 11/55591 in the name of the applicant company which describes a method and means for regulating an extruder of the roller tip type with the band obtained being laid directly onto a downstream device. According to that document, the rotational speed of the screw is directly proportional to the rotational speed of the roller and it is obtained by multiplying the roller speed by a proportionality coefficient the setpoint value of which is established beforehand experimentally. Control over the parameters of the band obtained at the outlet of the extruder is achieved by sensors coupled to the regulating means which act on the speed ratio between the roller and the extruder screw, correcting the proportionality coefficient setpoint value. This means that the throughput of the extruder screw can be synchronized at all times with that of the roller, while at the same time achieving a rapid response time for the regulating means.
The roller speed of the extruder described in that document dictates the rate at which the band is laid on the downstream device. Hence a problem arises when, under certain operating conditions, the efficiency of the screw drops, which implies that the regulating means need to command an acceleration in the rotational speed of the screw in order to keep the throughput constant. However, it has been found that, upwards of a certain screw speed value, the mixture inside the extruder suffers an increase in temperature. This causes gases to appear in the mixture, leading to bubbles in the band leaving the extruder and, sometimes, to degradation that may go so far as the formation of vulcanized agglomerations within the product leaving the extruder.
One solution to this problem is to measure the temperature of the compound inside the extruder or of the product leaving the latter, but that entails a delay in the regulation and leads to losses of time and of material.
Another solution is to permanently limit the speed of the laying roller to below a safe value, but at the expense of an obvious drop in the productivity of the extruder.
Indeed it has been established, during laboratory testing, that the efficiency of an extruder screw increases with the difference in temperature between the mechanical components of the extruder, which are at a high temperature as they are heated, and the strip of elastomer which is itself introduced continuously into the extruder at ambient temperature.
However, during a transient extruder stoppage, the heat energy from the components of the machine is transferred to the elastomeric compound and the temperature difference decreases. The consequence of this is that the efficiency of the screw decreases and when the extruder is restarted, it receives a command to accelerate the screw in order to compensate for the drop in efficiency. Such a situation leads to additional heating of the compound and causes the screw to run away.
The same thing happens if the extruder has an insufficient supply of elastomeric compound (for example if the strip of elastomer fed into the extruder breaks) when the throughput drops, the regulation control also dictates an acceleration of the screw, the consequence of this being that the screw runs away and the elastomeric compound is degraded.